The invention relates to a process for operating a regulated heat conduction vacuum gauge with a Wheatstone bridge powered by a controllable supply voltage and a gauge filament and a resistor as two of its components among others, designed to be temperature-dependant to compensate for interference effects of the ambient temperature on the gauge filament. Moreover, the invention relates to circuits suitable for implementation of this process.
Heat conduction vacuum gauges utilize the effect, that from a temperature-dependant resistance element, more heat is lost at high gas pressures, i.e. at higher particle densities, compared to lower gas pressures. In the heat conduction vacuum gauge after Pirani, the temperature-dependant resistance element is a gauge filament which is part of a Wheatstone bridge. In the unregulated Pirani vacuum gauge, a change in the resistance of the gauge filament unbalances the bridge whereby this imbalance is taken as a measure for the pressure. In the regulated Pirani gauge, the supply voltage which is applied to the bridge is continuously regulated in such a manner, that the resistance and thus the temperature of the gauge filament remains constant, irrespectively of the heat loss. The current required to maintain the resistance value at a constant level is a measure for the heat conduction and thus for the pressure of the gas. Commonly, the Wheatstone bridge is aligned for minimum imbalance by readjusting the supply voltage applied to the bridge accordingly. The bridge supply voltage thus represents the primary electrical quantity which corresponds to the pressure.
The ambient temperature of the gauge filament has an interfering effect on this measurement principle, since it also has an influence on the thermal equilibrium of the gauge filament and its surroundings via thermal radiation and thermal conductance. In order to compensate this interfering influence of the ambient temperature, it is known to include in one of the arms of the Wheatstone bridge a temperature-dependant resistance having a suitable characteristic. However, this kind of temperature compensation is inadequate, since it would have to depend with different characteristics on the pressure of the gas. Commonly, the characteristic is selected in such a manner that it is optimized for atmospheric pressure. At low pressures an incorrect compensation is thus unavoidable.